This invention relates to efficient information coding of interlaced scan digital video sequences for digital storage media or transmission media.
Standardization efforts of digital coding methods for video signals are actively progressing nowadays. Such standardization efforts include the studies of digital coding of various video signal formats at different bit-rates. In terms of scanning techniques which define how the video sequence is captured by recording devices and refreshed on display devices, video sequences can be classified into two types: progressive scan video sequences, and interlaced scan video sequences. In a progressive scan video sequence, a frame in the sequence is captured and refreshed sequentially line by line from top to bottom of the frame. An interlaced scan video sequence frame consists of two fields, the even field made up of the even lines of the frame and the odd field made up of the odd lines of the frame. Capturing and refreshing are performed first on the even field, sequentially from top to bottom of the field, and followed by the odd field in the same manner. Since a large number of present sources use the interlaced scan format (e.g. NTSC, PAL), a number of research efforts had been directed to efficient coding of the interlaced scan video sequence.
The prior research results can be classified into three categories: frame coding processes, field coding processes, and interpolated field coding processes. In a frame coding process, (For example in Kazuto Kamikura and Taikun Li, "Consideration of Bit Allocation and Distribution of Moving Image Coding for Storage Media", DAPA Meeting Report, Fourth Telematic Symposium, Tokyo, February 1991) a video sequence is basically coded frame by frame where the even and odd fields are combined in the interlaced manner (the frames are treated as if they were progressive). Each frame in a frame coding process can be partitioned into blocks of pixel data which are then processed by a block coding method such as Discrete Cosine Transform (DCT) Coding with or without Motion Compensation (MC) as described in MPEG Video Simulation Model Three, International Organization for Standardization, Coded Representation of Picture and Audio Information, 1990, ISO-IEC/JTC1/SC2/WG8 MPEG90/041. If a frame is coded independent of other frames in the sequence and without different treatments to the even field and the odd field, the process is called an intra-frame coding process; if the frame is coded with motion compensation using other frame(s) in the sequence, the process is called an inter-frame coding process. In summary, the frame coding process is a process whereby the video sequence is coded with just an intra-frame coding process, or a combination of intra-frame and inter-frame coding processes.
In a field coding process, a video sequence is first split into two sequences, the even field sequence and the odd field sequence, where each of the even field and odd field sequences is subjected to the same coding process. (See, for example, the paper by Ichiro Andoh, Y. Yamada et al., "Some Considerations and Experiments of Interlaced Moving Image Coding", DAPA Meeting Report, Fourth Telematic Symposium, Tokyo, February 1991.) Motion compensation or motion interpolation in a field coding process can be performed independently on each of the even field sequence and odd field sequence, or inter-dependently on the two sequences as illustrated by the paper by Ichiro Andoh et al. The field coding process, hence, is just an intra-field coding process or a combination of intra-field and inter-field coding processes.
As for the interpolated field coding process, only one of the field sequence is coded, and the other field sequence is reproduced by a prediction or interpolation process on the coded field sequence. Examples of this coding process are described in the paper by Feng-Ming Wang and Dimitris Anastassiou, "High-Quality Coding of the Even Fields Based on the Odd Fields of the Interlaced Video Sequences," IEEE Transection on Circuits and Systems, January 1991, and the MPEG Video Simulation Model Three, International Organization for Standardization, Coded Representation of Picture and Audio Information, 1990, ISO-IEC/JTC1/SC2/WG8 MPEG90/041.
With objects in the interlaced scan video sequence maintained stationary, correlation between adjacent pixels on every line and adjacent lines is high which allows popular image compression methods such as the conventional Discrete Cosine Transform Coding method, to achieve high compression ratio in a frame coding process (taking advantage of the high correlation between the lines) as compared of the high correlation between the lines) as compared to the field or interpolated field coding process. Inter-frame motion compensation can also be performed in this case with better efficiency. However, if objects in the sequence start to move, the correlation between the fields will drop, and hence the efficiency of the DCT method, for example, will be better if a field coding process is used. Furthermore, when motion compensation is involved in this situation, a inter-field coding process which performs the motion compensation individually on each fields has the advantage of better estimating the motion of the objects in the sequence. It can be seem that in general, the frame coding, field coding, and interpolated field coding process are sub-optimal in terms of efficiency given the fact that movement of objects in a sequence can be very dynamic. The improvement of efficiency of video coding systems is especially important in high bit-rate reduction coding systems.